Simulating a 40W Light Bulb
Simply convert Lumens to radiant flux.

Set Color to white, Power to 1W [link], and Size to 25 mm
Enter 450 Lumens (energy emitted by a 40W incandescent lightbulb)[link]
Divide the Lumens by 683 to get radiant flux [link] (which is used by Blender)

You can use Blackbody node to specify the 2700K color temperature.

Sources:

Cycles, unit of light energy ATTN: Brecht
Why Watt as light value - Comment by Troy_s
CG Cookie - How physically based, photometric lighting can improve 3D renders